The present invention relates to an operation control apparatus for recovery boilers in which black liquor produced in a pulp production process is treated to obtain smelted sodium sulfide Na.sub.2 S and high temperature steam.
Black liquor is produced as a waste liquor from a chip cooking step in the pulp production process. The black liquor contains sodium sulfate Na.sub.2 SO.sub.4 produced from inorganic matter such as sodium hydroxide NaOH used as chip cooking liquor and organic matter such as lignin included in the chip. When such a black liquor is heated and injected into a recovery boiler, the organic matter is burnt to recover a material for the cooking liquor and high temperature steam is simultaneously obtained as the output of the boiler. More particularly, when the black liquor is injected into the boiler, the droplet of the black liquor is dried while dropping in the boiler, and it is deposited as charbed on the bottom of the boiler furnace. When the charbed is burnt, a deoxidization reaction occurs to convert sodium sulfate Na.sub.2 SO.sub.4 contained in the charbed into sodium sulfide Na.sub.2 S, thereby recovering the smelted Na.sub.2 S from the bottom of the boiler furnace into the exterior and simultaneously generating a large quantity of high temperature steam to output it from the boiler.
In order to stably operate such a recovery boiler, it is understood to necessitate the controls of a number of factors such as the temperature and the pressure of the black liquor, the flow rate of air blown into the boiler, the depositing height of the charbed in the boiler, and the temperature distribution of the surface. Further, sulfur dioxide SO.sub.2 is contained in the gas exhausted from the recovery boiler, and it is also understood that there is a correlation between the boiler operating state and the concentration of the sulfur dioxide.
For example, if the diameter of the black liquor droplet increases due to a temperature or pressure drop of the black liquor or to an increase of the concentration of the liquor, the water content of the charbed increases causing the burning temperature of the charbed to decrease, and the deoxidization reaction of the sodium sulfide is disturbed. Further, the charbed abnormally increases in size to collapse and disturb the stable burning of the charbed.
On the contrary, if the diameter of the black liquor droplet decreases due to the temperature or pressure rise of the black liquor or due to the decrease of the black liquor concentration, it is burnt before depositing on the bottom of the boiler, then a very fine dust will be formed without the charbed being formed, and the deoxidization reaction of the sodium sulfide might not preferably take place.